This invention relates to a holographic recording medium and to the composition of this medium which provides for a condition of low volume shrinkage, good recording sensitivity, and high dynamic range.
In prior art processes for formation of volume-phase holograms, data is stored as holograms resulting from the interference of signal and reference beams within a holographic recording medium comprising a homogenous mixture of at least one polymerizable monomer or oligomer and a polymeric binder; the polymerizable monomer or oligomer must of course be sensitive or sensitized to the radiation used to form the interference fringes. In the illuminated regions of the interference pattern, the monomer or oligomer undergoes polymerization to form a polymer that has a refractive index different from that of the binder. Diffusion of the monomer or oligomer into the illuminated regions, with consequent chemical segregation of binder from these areas and alteration in its concentration in the non-illuminated regions, produces spatial separation between the polymer formed from the monomer or oligomer and the binder, thereby providing the refractive index modulation needed to form a hologram. Typically, after the holographic exposure, a post-imaging blanket exposure of the medium to actinic radiation is required to complete the polymerization of the monomer or oligomer and fix the hologram. When holograms are multiplexed co-locationally, such as by multiple holographic exposures at different angle conditions, a post-imaging blanket exposure of the medium to actinic radiation may also be required to complete the polymerization of the monomer or oligomer and fix the multiplexed holograms.
One important potential use of volume holograms is in digital data storage; the three dimensional nature of a volume hologram, which refers to the storage of each bit as a hologram extending throughout the entire volume of the recording medium, renders volume holograms suitable for use in high capacity digital data storage. A group of bits can be encoded and decoded together as a two dimensional array of bits referred to as a page. Various multiplexing methods, such as angular, spatioangular, shift, wavelength, phase-code, and related methods, are used to store multiple pages co-locationally within the same volume or in partially overlapping volumes.
Photopolymerizable holographic recording media for write-once-read-many (WORM) data storage applications should ideally exhibit pre-recording shelf life of at least a year, good recording sensitivity, high degree of optical homogeneity (i.e. low scattering), uniform recording characteristics, stable image fidelity, and low volume shrinkage coupled with high dynamic range or cumulative grating strength. Low volume shrinkage coupled with good dynamic range and recording sensitivity, however, remains as one of the most difficult to achieve performance characteristics for photopolymerizable holographic recording media that are designed for data storage applications. Typically, high dynamic range is achievable with photopolymer recording materials but the resultant volume shrinkage is significant and thus poor image fidelity and poor signal to noise results. Uh-Sock Rhee et al. in Applied Optics, 34, 5, 846 (1995) describe the shrinkage effect in Dupont HRF-150-38 photopolymer as a function of increasing exposure. This material comprises monomers that are photopolymerized using conventional free radical polymerization chemistry. The magnitude of the deviation of the Bragg angle, displayed in FIGS. 6 and 7 of the above reference, is about 2.5°. This value is five times larger than the full width at half maximum (FWHM) of the Bragg peak for a plane-wave hologram with a grating period in the intermediate range of a digital page based image, and which is recorded in a medium thickness of only 100 μm. Accordingly, even for such a thin recording medium, which is not sufficiently thick for a holographic data storage medium, the angle shift from the recording condition exhibited by the Dupont HRF-150-38 material is so large that no diffraction efficiency is observed at the recording angle condition, and thus an image could not be reconstructed without substantially tuning the angle of the read beam. Tuning the angle of the read beam is not desirable for a holographic data storage system since this imposes significant overhead on the readout design and would seriously impair data rates. Furthermore, a single tuning adjustment would be inadequate to read an image affected by such levels of shrinkage, since page based images comprise a continuum of plane-wave grating components with a range of grating angles. Contributions to an image from larger grating angle components would be shifted further in angle from the Bragg condition than for smaller grating angle components, and thus differential tuning would be required to reconstruct the high and low frequency features of an image. For thicker recording media this problem is exacerbated further due to the inverse proportionality of FWHM on media thickness.
A consequence of high volume shrinkage is the requirement to use up a portion of the reactive monomeric or oligomeric species in the recording medium before holographic recording of information can be implemented with reasonable fidelity. This step has the effect of diminishing the usable dynamic range as well as causing significant decline in recording sensitivity. An example of this tradeoff is the photopolymer material developed by Lucent Technologies, and which is based upon conventional free radical polymerization chemistry. By way of example, this material comprises difunctional acrylate oligomers and monomers such as N-vinyl carbazole and isobornyl acrylate (see Dhar et al. in Optics Letters, 23, 21, 1710 (1998), all of which exhibit significant volume shrinkage upon polymerization. Accordingly, Dhar et al. describe that if more than twenty 480-kbit images are recorded co-locationally in this material, for either 250 or 500 μm thick media, then the raw bit-error rate (BER) rises above the desired upper limit value of 5e−3. This occurs due to excessive volume shrinkage, which firstly reduces image fidelity and secondly causes an increased degree of Bragg detuning as the extent of polymerization increases. In Optics Letters, 24, 7, 487 (1999), Dhar et al. describe a photopolymer recording media that exhibits moderate dynamic range per unit thickness but which suffers from significant volume shrinkage. Accordingly, Dhar et al. show that the cumulative grating strength for a material that exhibits 0.5% transverse shrinkage declines by a factor of about 4 to 5 when the concentration of monomer is reduced in order to achieve an improved and minimally desirable value of 0.2% transverse shrinkage. Consequently, for 200 μm thick media Dhar describes that the cumulative grating strength diminishes from about 9 to about 2, an unacceptably low value, when a preconditioning step is used to consume a portion of the monomeric species in order to reduce shrinkage during holographic recording. Additionally, the recording sensitivity of the recording material declines when the monomer concentration is reduced to compensate for shrinkage, as the physical state of the material during recording more closely resembles a glassy polymer in which diffusion rates are substantially reduced. Accordingly, despite Dhar et al. having prepared such materials with increased thicknesses, up to about 1 mm, in order to compensate for the serious decrease in dynamic range exhibited at acceptable levels of transverse shrinkage, the recording sensitivity was still low. When holographic recording media are prepared with increased media thickness, however, then the degree of Bragg detuning, exhibited for any given level of transverse shrinkage, becomes more problematic as a result of the concomitant diminution in the peak width of the Bragg selectivity angular profile. In particular, when the magnitude of Bragg detuning is large relative to the Bragg selectivity peak width, then the observed diffraction efficiency is substantially lower than the value at the Bragg peak, and thus the raw BER of a reconstructed image increases significantly as a result of decreased signal to noise ratio.
In prior art, holographic recording media based upon cationic ring opening polymerization have employed monomers which when homopolymerized produce hard and brittle polymers due to high crosslink density. High crosslink density can act to inhibit attainment of significant extents of polymerization reaction. This is particularly the case for multifunctional moieties where enthalpy values for homopolymerizations can be less than about 50 (kJ/mole epoxide). Previous compositions comprising such multifunctional monomers are described by way of example in U.S. Pat. No. 5,759,721 and in U.S. patent application Ser. No. 08/970,066. Although these monomers exhibit considerably reduced shrinkage upon polymerization, as compared to other monomers such as acrylates, the remaining shrinkage coupled with their relatively unyielding mechanical properties can cause both mechanical and optical difficulties when employed in holographic recording media with thickness of 200 μm or greater. Additionally, inhibition of significant extents of polymerization reaction due to high crosslink density prevents attainment of the full dynamic range of the photopolymerizable medium during holographic recording.